Aside from the glaringly obvious fact that you’re not consuming any calories when you fast, does it work? Is it a good way to lose weight? Does it have health benefits? That depends. Let me explain.
There’s a lot of talk out there about the health and weight loss benefits of fasting, but not a whole lot of scientific data (unless you’re a rodent). Healthy caloric restriction is beneficial in terms of both a longer life and a healthier life (1). But does fasting have any particular benefits over a continuous low-calorie, healthy diet? I do think that there’s enough solid information in recent studies to make some recommendations (2). Here are the three I’ll talk about today:
1. Alternate-Day Fasting
2. Intermittent Modified Fasting
3. Time-Restricted Eating
1. Alternate-Day Fasting
In this model, you consume zero calories every other day. Then, on alternate days, there's no restriction on calories. The biggest problem with this plan may be a very simple one: hunger. Some people are really hungry on fast days (big surprise, right?), and this may not decrease with time (3). This hunger state is bound to lead to a feeling of deprivation, which simply cannot be sustained. Moreover, this strategy is associated with only mild weight loss and no improvement in glucose metabolism (4). There’s another problem, too: there’s some evidence that mood and perceived performance may diminish with alternate-day fasting (5).
Dr. J’s Verdict: Pretty Stupid
2. Intermittent Modified Fasting
This regimen refers to decreasing caloric intake on fasting days, usually to about 25% that of non-fasting days. The popular “5:2,” or “Fast Diet” is based on this model. The majority of studies evaluating this and other fasting regimens have not found any health or weight loss benefits greater than with general caloric restriction. (6,7). Also, persistent hunger may be a problem with this regimen, too (8).
Dr J’s Verdict: Maybe.
Okay, now we’re getting somewhere. Time-restricted eating refers to avoiding food later at night or closer to bedtime. Why would this matter? The answer is that it’s not just what you eat, but when you eat it. The basis for time-restricted eating is to synchronize caloric intake with the body’s circadian rhythms (9). The body’s hormones (10) and intestinal microbiome (11) respond to these rhythms. Insulin levels, in particular, are lower at night. The body simply doesn’t metabolize glucose as well later in the day — and this increases the risk for obesity, diabetes, heart disease, even if the overall calorie intake isn’t increased (12). Other hormones are affected as well, which may partially explain why people who eat more at night may be at increased risk for cancer, particularly breast cancer (13).
Nighttime eating is associated with reduced sleep quality and duration (14) — and we know how deleterious sleep deprivation is to physical and mental health, emotional well-being, and weight control. Finally, people who eat later at night tend to consume more calories than those people whose major calorie consumption occurs earlier in the day (15).That comes as no surprise: let’s face it — no one craves celery at ten-thirty at night.
To sum it up, while there aren’t many studies on humans yet, the results of this time-restricted eating are promising regarding both weight loss and improved metabolic profile — and with no decrease in mood or performance (4).
Dr. J’s Verdict: Super!
Bottom Line: If you’re going to restrict calories, do it in a way that feels good and is sustainable. There’s good evidence that eating most of your food earlier in the day is good for you. All the rest is commentary.
1. Lettieri-Barbato D et al. Effects of dietary restriction on adipose mass and biomarkers of healthy aging in human. Aging-US 2016; 8(12):3341-55.
2. Patterson RE; Sears DD. Metabolic effects of intermittent fasting. Ann Rev Nutr 2017: 37:371-93.
3. Heilbronn LK et al. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Am J clin Nutr 2005; 81:69-73.
4. Varady KA. Impact of intermittent fasting on glucose homeostasis. Curr Opin Clin Nutr & Metab Care 2016; 19(4):300-2.
5. Appleion KM, Baker S. Distraction, not hunger is associated with lower perceived work performance on fast compared to non-fast days during intermittent fasting J Hlth Psychol 2015; 20:702-11.
6. Harvie MN, Howell T. Could intermittent energy restriction and intermittent fasting reduce rates of cancer in obese, overweight, and normal-weight subjects? A summary of evidence. Adv Nutr 2016; 7:690-705.
7. Headland M et al. Weight-loss outcomes: a systematic review and meta-analysis of intermittent energy restriction trials lasting a minimum of 6 months. Nutrients 2016; 8:354.
8. Johnstone A. Fasting for weight loss: an effective strategy or latest dieting trend? Int J Obes 2015; 39:727-33.
9. Longo VD, Panda S. Fasting, circadian rhythms, and time-restricted feeding in healthy lifespan. Cell Metab 2016; 23:1048-59.
10. Gamble KL et al. Circadian clock control of endocrine factors. Nat Rev Endocrinol 2014; 10:466-75.
11. Kaczmarek JL et al. Time of day and eating behaviors are associated with the composition and function of the human gastrointestinal microbiota. Am J Clin Nutr 2017; 106:1220-31.
12. Bo S et al. Consuming more of daily caloric intake at dinner predisposes to obesity: a 6-year population-based prospective cohort study. PLOS ONE 2014; 9:e108467.
13. Grundy A et al. Increased risk of breast cancer associated with long-term shift work in Canada. Occup Environ Med 2013; 70:831-8.
14. Yamaguchi M et al. Relationship of dietary factors and habits with sleep-wake regularity. Asia Pac J Clin Nutr 2013; 22:457-65.
15. Reid KJ et al. Meal timing influences daily caloric intake in healthy adults. Nutr Res 2014; 930-5.